1. Field of the Invention
This invention relates to an electroluminescence (EL) element, and to a method of driving an electronic display manufactured by forming thin-film transistors (hereinafter abbreviated as TFTs) on a substrate. Particularly, the invention relates to a light-emitting device which uses semiconductor elements (elements formed from a semiconductor thin film). The invention further relates to electronic devices using the light-emitting device as a display unit.
In this specification, the EL elements include the ones which utilize emission of light from singlet excitons (fluorescence) and the ones which utilize the emission of light from triplet excitons (phosphorescence).
2. Description of the Related Art
In recent years, light-emitting devices having EL elements have been vigorously developed as self light emitting elements. Unlike the liquid crystal display devices, the light-emitting device is self light emitting type. The EL element has a structure in which an EL layer is held between a pair of electrodes (anode and cathode).
The light-emitting devices can include those of the passive matrix type and those of the active matrix type. Here, the devices of the active matrix type are suited for the applications where a high-speed operation is required for an increase of pixels accompanying an increase in the resolution and for movie display.
Each pixel in the organic EL panel of the active matrix type is provided with a holding capacitance (Cs) for holding the voltage. FIG. 12A illustrates an actual example of pixel constitution, and FIG. 12B is an equivalent circuit thereof. As disclosed in patent document 1, the capacitance Cs tends to be large and the light-emitting area of organic EL tends to be small correspondingly. In addition to the capacity Cs, shapes, numbers and arrangements of TFTs, wirings, contacts, partitioning walls and the like constituting pixels become factors for decreasing the light-emitting areas. As the light-emitting area decreases, the current density increases and the reliability of the organic EL element decreases seriously.
(Patent Document 1)
Japanese Patent Laid-Open No. Hei 8-234683
Further, if an opening portion is formed in a complex shape in order to increase aperture ratio as much as possible, a shrink of the organic EL portion may be promoted. Here, the shrink of the EL portion is not a state where the EL layer physically shrinks, but a state where the effective area of the EL element (area of a portion where the EL element emits light) gradually shrinks starting from the end portions. Namely, as the shape of the opening portion becomes complex, the length of the end portion increase relative to the area of the opening portion promoting the shrink.
FIG. 20 illustrates an example of the constitution of a pixel portion in an active matrix type EL display device. A portion surrounded by a dotted line frame 2300 stands for a pixel portion which includes a plurality of pixels. A portion surrounded by a dotted line frame 2310 stands for one pixel.
Gate signal lines (G1, G2, - - - , Gy) to which selection signals are input from a gate signal line drive circuit, are connected to gate electrodes of switching TFTs 2301 included in the pixels. Further, one of a source region and a drain region of the switching TFT 2301 included in each pixel is connected to a source signal line (Sl to Sx) to which signals from the source signal line drive circuit are input, and the other one is connected to the gate electrode of the driving TFT 2302. One of the source region or the drain region of the driving TFT 2302 included in each pixel is connected to a current supply line (V1, V2, - - - , Vx), and the other one is connected to one electrode of the EL element 2304 included in each pixel. Further, each pixel may be provided with capacitance means 2303 for holding a voltage between the gate and the source of the driving TFT 2302 during a display period.
The EL element 2304 has an anode, a cathode and an EL layer provided between the anode and the cathode. When the anode of the EL element 2304 is connected to the source region or the drain region of the driving TFT 2302, the anode of the EL element 2304 works as a pixel electrode and the cathode thereof works as an opposing electrode. Conversely, when the cathode of the EL element 2304 is connected to the source region or the drain region of the driving TFT 2302, the cathode of the EL element 2304 works as a pixel electrode and the anode thereof works as an opposing electrode.
In this specification, the potential of the opposing electrode is called opposing potential. A power source which gives an opposing potential to the opposing electrode is called opposing power source. A difference between the potential of the pixel electrode and the potential of the opposing electrode is an EL drive voltage. The EL drive voltage is applied to the EL layer held between the pixel electrode and the opposing electrode.
As a gradation display method for the light-emitting device, there can be exemplified an analog gradation system and a digital gradation system.
Next, described below are the values of when Cs is provided in the cases of the analog gradation system and the digital gradation system.
In the case of the analog gradation system, in general, an analog video signal is written into each pixel once in one frame period. The analog video signals are input to the pixels in the form of an analog voltage or an analog current. In the case of the analog voltage, the analog voltage that is written is stored in the holding capacitors of the pixels for one frame period (one frame period lasts 16.66 ms when the frame frequency is 60 Hz). In the case of the analog current, the current that is written is once converted into an analog voltage in the pixels. The analog voltage must be maintained for one frame period.
In the case of the digital gradation system, as described above, the digital video signal must be written a plural number (n) of times in one frame period. In the case of the 4-bit gradation, n=4 times or more times and in the case of the 6-bit gradation, n=6 times or more times. Therefore, the analog voltage must be maintained for a period of the longest sub-frame among the n sub-frames divided from one frame period.
Next, described below is a relationship between the driving TFT and the EL element.
Referring to FIG. 15A, a driving TFT 1505 and an EL element 1506 are connected in series between the current supply line and the opposing power source in each pixel. As for the current flowing into the EL element 1506, a point where the Vd-Id curve of the driving TFT intersects the V-I curve of the EL element in FIG. 15B becomes an operation point. Electric current flows depending upon a voltage between the source and the drain of the driving TFT 1505 and upon a voltage between the electrodes of the EL element 1505.
When the gate-source voltage (|VGS|) of the driving TFT 1505 is greater than the source-drain voltage (|VDS|) by more than a threshold voltage, the driving TFT 1505 operates in a linear region (driving on a constant voltage). When the gate-source voltage (|VGS|) of the driving TFT 1505 is smaller than the source-drain voltage (|VDS|), the driving TFT 1505 operates in a saturated region (driving on a constant current).
When the driving TFT 1505 is operated in the linear region, namely, when the operation of the driving TFT 1505 at the operation point is included in the linear region, |VDS| of the driving TFT 1505 becomes very smaller than the voltage (|VEL|) across the electrodes of the EL element 1506, and variation in the characteristics of the driving TFT 1505 does not almost affect the current that flows through the EL element 1506. However, if the resistance of the EL element 1506 varies due to a change in the temperature or aging, the electric current is affected thereby and undergoes a change. When, for example, the EL element 1506 is degraded, and the voltage-current characteristics thereof change from 1601 to 1602 as shown in FIG. 16A, the operation point, too, shifts from 1603 to 1604. Here, when the driving TFT 1505 is operating in the linear region, the current flowing through the EL element 1506 decreases by ΔID accompanying the shift of the operation point. The brightness, therefore, decreases.
When the driving TFT 1505 is operated in the saturated region, on the other hand, the drain current (IDS) of the driving TFT 1505 remains constant as shown in FIG. 16B despite the voltage-current characteristics of the EL element 1506 have changed from 1611 o 1612 due to degradation of the EL element. Despite the operation point has changed from 1613 to 1614, therefore, a constant current flows into the EL element 1506. Accordingly, a change in the brightness is smaller than that of when the driving TFT 1505 is operated in the linear region.
The operation points can all be brought into the saturated region by setting the channel length and channel width of the driving TFTs and by selecting the characteristics and driving voltages of the driving TFTs and EL elements.
When the driving TFT 1505 is operated in the saturated region, however, the current that flows into the EL element 1506 is determined solely by the VGS-IDS characteristics only of the TFT. Therefore, the variation in the brightness of the EL element 1506 is reflected by the variation in the characteristics of the driving TFT 1505. Further, the electric current is seriously affected by a change in the gate-source voltage VGS during the holding period. The drain current IDS in the saturated region is expressed by the formula (1),IDS=β/2×(VGS−|Vth|)2  (1)
Due to an off-leak current of the switching TFT 1504, the electric charge on the gate electrode of the driving TFT 1505 leaks into the source signal line 1501, and the gate-source voltage |VGS| of the driving TFT changes correspondingly resulting in a change in the drain current IDS. Therefore, a capacitor is necessary for compensating the loss of gate-source voltage VGS of the driving TFT caused by the leak of the electric charge from the switching TFT 1504. This is called holding capacitance. The magnitude of the holding capacitance is determined by a relationship between the VGS-IDS characteristics of the driving TFT and the amount of change ΔIEL in the current that accompanies a change of brightness of the EL element 1506 by one gradation. As will be understood from the formula (1), the drain current IDS varies in proportion to the second power of VGS. Therefore, a change in the drain current IDS is very susceptible to a change in the gate-source voltage |VGS|. From ΔIEL, the amount of change ΔVGS in the gate-source voltage VGS allowed for the driving TFT 1505 is obtained. The required magnitude of the holding capacitance is determined from the off-leak current IOFF of the switching TFT and the holding time by using the formulas (2) and (3),IOFF=CΔVGS/Δt  (2)Cs=IOFF×Δt/ΔVGS  (3)where Δt is a very short period of time and ΔVGS is an increment of the gate-source voltage of the driving TFT 1505.
In contrast with the digital gradation system which effects the writing operation a plural number of times per a frame period, the analog gradation system permits the writing operation to be done only one time in one frame. Therefore, the holding time becomes long and a larger holding capacitance is necessary.
Due to the above reasons, further, the channel lengths of the driving TFTs must be maintained long in the pixels. Further, the aperture ratio decreases as the size of the driving TFT increases.